In many radios, and in particular in radio telephones, the clock oscillator is a crystal oscillator with a relatively low frequency whose harmonics can occur in the reception band of the receiver.
GSM devices operate, for example, in the 900 MHz band. In conventional embodiments, the receiver has an intermediate frequency of 45–400 MHz and the transmitter operates with direct modulation of a carrier which is generated on the transmission frequency. A channel oscillator and a fixed frequency oscillator both use a 13 MHz crystal oscillator as a reference. The frequency of 13 MHz is used because the clock frequency which is required centrally by the GSM digital part relates to 13 MHz with 13/6=2.1666 MHz, and the channel raster frequency of 200 kHz, which is equal to the comparison frequency of the channel synthesizer, is also derived from it by simple frequency division. 13 MHz is thus the lowest possible reference oscillator frequency of each conventional GSM telephone.
The seventy-second harmonic of the 13 MHz oscillator occurs in the 936 MHz reception channel, and the seventy-third harmonic occurs in the 949 MHz reception channel. In mobile phones which are highly miniaturized, the spatial proximity of the radio modules to the receiver input and to the antenna results in a particular problem. Current specifications prescribe that interference frequencies at the 50 ohm receiver input must be less than 0.7 micro volts, for example. A customary 13 MHz crystal oscillator oscillates at an amplitude of approximately 1 volt, which means that harmonics of this oscillator may appear damped by more than 120 dB in the 900 MHz region at the receiver input. However, the high-speed silicon transistors which are customary today give rise to upper harmonic intervals of only approximately 60 dB in relation to the useful carrier in the 900 MHz region. Additional damping between the crystal oscillator and the receiver input of approximately 60 dB is thus necessary. Given the small spatial distances between these regions of at most 40 mm, this gives rise to an extreme level of expenditure on shielding in customary GSM mobile phones, i.e. to the use of sheet metal parts, metal housing etc. Attempts at solving this harmonic problem by means of switching measures alone, for example by means of harmonic filters, have only been partially successful because the harmonics are generally reflected by them and not destroyed, which in an extreme case can even lead to a situation in which critical harmonics are irradiated in an amplified form.